1. Field of the Invention
The present invention generally relates to the field of antennas that have physical dimensions that are small compared to the wavelength of the electromagnetic radiation of interest. Such antennas are known as "small" antennas. The present invention more particularly relates to a small ferrite-core antenna for a radio receiver operating in the VHF to UHF range.
2. Background Art
When an electrically small loop of conductive material is used as a receiving antenna, the open-circuit voltage developed at the terminals of the loop is proportional to the component of the incident magnetic flux density normal to the plane of the loop. It is known that this open-circuit voltage can be increased by filling the loop with a core of magnetically permeable material, such as a ferrite. The effect of the core is to increase the magnetic flux through the area of the loop.
For small radio receivers operating in the AM broadcast band (at about 1 MHz) the ferrite rod inductor is the antenna most commonly used. An elongated ferrite rod in a thin helical coil greatly increases the effective area of the coil which intercepts the electromagnetic radiation. In this case, the "capture area" is determined primarily by the length of the ferrite rod rather than the diameter of the coil.
The number of turns in the coil is typically selected to set the impedance level at the terminals of the coil. In order to tune the antenna, a variable capacitor is typically connected in parallel across the terminals of the coil. Then the antenna resonates at the frequency at which the magnitude of the inductive reactance of the coil equals the magnitude of the capacitive reactance of the variable capacitor. The selection of the electrical parameters of the ferrite rod inductor antenna are further described in R. C. Johnson and Henry Jasik, Antenna Engineering Handbook, 2nd ed., McGraw-Hill Book Co., New York (1984) pp. 5-6 to 5-9.